Power

So far in this course, you have learned that it is necessary to create pressure on the electrons in a conductor to make them flow. This pressure is known by three different terms: potential, electromotive force (emf), and most commonly, voltage.

You have also learned that electron flow is commonly known as current. The more voltage, or pressure, that is present in an electrical circuit, the more electrons that will flow.

The product of the current times the voltage applied is power. Another way to define electrical power is the rate at which electrical energy is used. For example, you consume twice as much energy (power) if you keep a lamp on for two hours, rather than one hour. You would also consume twice as much energy (power) if you used two lamps for one hour, rather than one lamp for one hour.

The key point to understand is that power (P) is directly related to both the voltage (E) across a circuit and the current (I) flowing in that circuit. That relationship is expressed by the formula: P = EI.

Power is measured in units called watts. A watt is the rate at which energy is used when one volt produces a current of one amp.

To show you how to calculate the power, in watts, of a circuit, take a look at the schematic in Figure 2.19.

As you can see, this circuit features a 10-volt power supply connected to a 10-Ω resistor. According to Ohm's Law, the resulting current flow is 1 amp.

Figure 2.19

Since P = EI, that circuit consumes 10 watts of power. Actually, the power is not "consumed," but is "converted" from electrical energy into another form of energy, such as heat, light, motion, or a combination of the three.

For example, a 60-watt light bulb converts 60 watts of electrical power per hour into light and heat. Obviously, a 100-watt light bulb uses almost twice the power of a 60-watt light bulb. Naturally, this power is consumed, or converted, in a circuit only when current is flowing.

Now, why is it important for you to understand the relationship between watts, volts, and amps?

The main reason is that Bourns makes resistors, and resistors are specified in terms of Ωs and watts. Why is this the case? The answer is quite simple.

We have no way of knowing how much current a user intends to run through a resistor, and how much voltage will be used to do it. Bourns does know the number of watts that the resistor can handle and indicates it. Bourns expects the user to know how many watts each resistor will have to handle, and to select the correct one.

If the user does not know how to determine the circuit's requirements, you may have to provide some assistance. So remember the formula: P = EI.

In the next module of this training program you will learn a great deal more about resistors.